This invention relates generally to methods and apparatus for injection molding of synthetic resinous materials. More particularly, this invention concerns methods and apparatus for vented injection molding of articles from synthetic nylon materials in continuous cyclical production.
The successful injection molding of degradable synthetic resinous materials has long presented a problem to those interested in making molded products in continuous commercial production. Among the undesirable characteristics manifested by degradable materials are hygroscopicity and oxidation due to a reaction with atmospheric oxygen.
When deaing with hygroscopic materials in common commercial practice, the materials in particulate form are passed through a separate predrying apparatus before entering molding apparatus. In the predrying apparatus the particulate material is exposed to dry heat for a predetermined period of time at a temperature below the material melting temperature. This predrying has been necessary since water, present in the particulate material, vaporizes during material plasticization and generates pockets of water vapor. The pockets of water vapor, as well as other volatilized products, result in imperfections in the molded product.
To reduce water absorption from humid air, the particuate material is sometimes packaged and stored in the bags until actually supplied to an injection molding machine. In this manner the time during which air can affect the material is substantially reduced.
In an effort to overcome some of the disadvantages associated with production of molded objects from hygroscopic synthetic resinous materials, the use of vented injection molding machines has been suggested. The intended advantage of vented injection molding machines is the elimination of the material pedrying step. A reduction in the cost, as well as time, involved in molding products might be possible. However, with vented injection molding machines handling degradable synthetic resinous materials, different problems have manifested themselves.
One of the problems has concerned the discharge or bleed of material from the vent section itself, that is, the molten and plasticized material has a tendency to pass outwardly of the barrel through the vent openings through which water vapor and volatile gases are intended to be exhausted.
Another more particular problem concerns the length of time during which the satisfactory molded articles may be produced. With the known vented injection molding machines it has been found that only a small number of hours of continuous running is available during which acceptable molded products are fabricated from atmospherically degradable materials, such as nylons. After running a small number of hours, the molded product does exhibit black specs resulting from high temperature oxidation. Moreover, it is necessary after a small number of hours to dismantle the injection molding machine and clean residue from the screw which is mounted therein.
Accordingly, it is seen that a need continues to exist for a truly effective vented injection molding machine which is capable of producing molded articles from atmospherically degradable synthetic resinous materials, such as nylon.